1. Field of Invention
This invention relates generally to single-frequency ring lasers and more particularly to tunable single-frequency solid-state lasers.
2. Description of Prior Art
Ultra-stable, single-longitudinal-mode lasers are the heart of coherent optical transmitters and receivers in coherent communication. They are also useful as local oscillators in high-powered lasers and laser amplifiers. These lasers are an essential part in coherent optical range finders, designators, sensors, and other measurement instruments. In coherent optical rangefinders, for example, it is very important to force such a laser to oscillate in a purely longitudinal mode and to tune its frequency rapidly over a limited frequency range in order to detect and determine the location and velocity of remote objects, using heterodyne FM-CW chirped techniques. Such techniques are well-known in FM-CW chirped radar applications, and are described in the literature, for example, by Skolnik, Introduction to Radar Systems, 2nd ed., Chapter 3, McGraw-Hill, New York, 1980.
Recent developments describe the use of solid-state lasers pumped by semiconductor laser diodes which provide very small, highly efficient, single-mode ultra-stable solid-state lasers. Such solid-state lasers are described, for example, in U.S. Pat. No. 4,578,793; Kane and Byer, "Monolithic, Unidirectional Single-Mode Nd:YAG Ring Laser," Optics Letters 10:65-67 (Feb. 1985); Kane et al., "Frequency Stability and Offset Locking of a Laser-Diode-Pumped Nd:YAG Monolithic Nonplanar Ring Oscillator," Optics Letters, 12:175-177 (March 1987); and Trutna et al., "Unidirectional Diode-Laser-Pumped Nd:YAG Ring Laser with a Small Magnetic Field," Optics Letters 12:248-250 (April 1987).
In the prior art, single-piece, solid-state non-planar as well as quasi-planar, internally reflecting, unidirectional ring lasers pumped by semiconductor lasers, have been shown to emit highly coherent single-frequency laser light. The advantage of the prior-art design configuration is its stability and simplicity due to the use of a single piece of solid-state laser material with magneto-optic properties and polished surfaces that create a unidirectional ring path for the laser beam. The internally reflective surfaces act to change the direction of the linear-polarized lasing light in order to compensate for the plane of polarization change induced by the Faraday rotation due to an externally applied magnetic field. Since the magneto-optic effect is non-reciprocal, the plane of polarization of a beam that traverses the ring in one direction, e.g. counter-clockwise, is designed to be aligned with the original polarized beam after completing one round. On the other hand, the beam which traverses the ring in the opposite direction, e.g., clockwise, increases its polarization rotation each time it completes a round trip in the cavity. A polarization-selective element in the cavity, aligned with the compensated, counter-clockwise, linear-polarized beam, induces differential losses between the two opposite traveling waves. The induced differential loss between the two contra-rotating beams occurs by use of a polarization-selective element, since one of the rotating beams (the clockwise beam in this example) has a plane of polarization which accumulates increasing deviation from the aligned polarizer orientation and thus an increased loss, each time the beam completes a full path within the cavity. On the other hand, for the polarization-compensated beam (in this example, the counter-clockwise beam) the plane of polarization remains aligned with the polarizer orientation, so that the beam does not suffer significant loss. The differential losses reinforce the counter-clockwise beam over the clockwise rotation, thereby creating a unidirectional traveling-wave lasing. The traveling-wave lasing configuration saturates the gain uniformly, thereby eliminating spatial hole-burning that creates multifrequency operation in a linear Fabry-Perot-type cavity. The gain is uniformly saturated since the electric field of the electromagnetic wave is travelling through the pumped volume. This is in contrast to the action in a Fabry-Perot configuration in which stimulated atoms spacially located across the nodes of the standing waves form unsaturated gain, in which case other longitudinal modes having different frequencies tend to oscillate and saturate this unsaturated gain.
The prior-art monolithic design, for example as shown in U.S. Pat. No. 4,578,793, and in Kane and Byer, "Monolithic, Unidirectional Single-Node Nd:YAG Ring Laser," Optics Letters, vol. 10, No. 2 (Feb. 1985), pp. 65-67, gives a highly stable single-frequency laser output. However, the stable monolithic design is inferior when fast frequency tuning is needed. Slow frequency tuning can be applied to the monolithic design by heating or cooling the solid-state lasing material. Fast frequency tuning over a very limited spectral range can be achieved by applying a variable pressure on the lasing material, such as Nd:YAG, using a piezo-electric transducer (PZT). See also A. Owyoung and P. Esherick, Optics Letters, vol. 12, p. 999 (1987), and T. J. Kane, Lightwave Electronics Corporation, Mountain View Calif., "Demonstration of Two Frequency-Locked Ring Lasers," exhibition shown at the Conference on Laser and Electro-Optics, Apr. 26-28, 1988, Anaheim, Calif.
Also known in the prior art is a co-linear pumping technique where a laser diode is tuned to emit light in the exact absorption band of the solid-state laser material, as described, for example, in Kane et al, "Frequency Stability and Offset Locking of a Laser-Diode-Pumped Nd:YAG Monolithic Non-Planar Ring Oscillator," Optics Letters, vol. 12, no. 3 (March 1987), pp. 175-177. The laser diode light is focused on the same axis of the propagating light inside the cavity, colinear with the lasing beam. In the colinear pumping configuration the pumped volume overlaps exclusively with the lowest-order transverse-cavity mode, which gives rise to a lasing of single transverse mode, namely the TEM.sub.00 gaussian mode. Other prior art, pertaining to monolithic solid-state lasers pumped by laser diodes, is described by Trutna et al., "Quasiplanar Unidirectional Ring Laser," Paper WN2, presented at the Conference on Laser and Electro-Optics, Apr. 26-29, 1987, Baltimore, Md., and by Trutna et al., "Unidirectional, Diode-Laser-Pumped Nd:YAG Ring Laser with a Small Magnetic Field," Optics Letters, vol. 12 (April 1987), p. 248. This design has better performance due to exact compensation of the polarization plan rotation by the total internal reflecting facet of the prism.